Question: How to determine if a value is a valid nil/empty string or is absent in the input JSON. If the value is absent, it shouldn't be changed, but if it's present and is nil/empty string it should be removed.

example.go

package example

const (
	updateJson1 = `
{
    "firstname": "Peter",
    "lastname": "Griffin"
}
`

	updateJson2 = `
{
    "address": {
        "street": "31 Spooner Street",
        "city": "Quahog",
        "country": "United States of America"
    }
}
`

	updateJson3 = `
{
    "address": null
}
`
)

var (
	person = &Person{
		Firstname: "Lois",
		Lastname:  "Griffin",
		Address: &Address{
			Street:  "31 Spooner Street",
			City:    "Quahog",
			Country: "United States of America",
		},
	}
)

type Address struct {
	Street  string
	City    string
	Country string
}

type Person struct {
	Firstname string
	Lastname  string
	Address   *Address
}

// Update the firstname and lastname
func update1() {
	var updatePerson *Person

	json.Unmarshal(updateJson1, &updatePerson)

	person.Firstname = updatePerson.Firstname
	person.Lastname = updatePerson.Lastname

	// The address is absent in this example, thus shouldn't be changed.
	// I can't check for nil, since nil as a valid value.
	if false {
		person.Address = updatePerson.Address
	}
}

// Update the address
func update2() {
	var updatePerson *Person

	json.Unmarshal(updateJson2, &updatePerson)

	// The Firstname is absent in this example, thus shouldn't be changed.
	// I can't check for empty string, since empty string is a valid value.
	if false {
		person.Firstname = updatePerson.Firstname
	}

	// The Lastname is absent in this example, thus shouldn't be changed.
	// I can't check for empty string, since empty string is a valid value.
	if false {
		person.Lastname = updatePerson.Lastname
	}

	person.Address = updatePerson.Address
}

// Remove the address
func update3() {
	var updatePerson *Person

	json.Unmarshal(updateJson3, &updatePerson)

	// The Firstname is absent in this example, thus shouldn't be changed.
	// I can't check for empty string, since empty string is a valid value.
	if false {
		person.Firstname = updatePerson.Firstname
	}

	// The Lastname is absent in this example, thus shouldn't be changed.
	// I can't check for empty string, since empty string is a valid value.
	if false {
		person.Lastname = updatePerson.Lastname
	}

	// The address is nil, so it should be removed.
	person.Address = updatePerson.Address
}